Lightweight sandwich panel structures consisting of low density cores and solid facesheets are widely used in engineering applications. Cellular core structures based upon honeycomb topologies are often used because of their high compressive strength-to-weight ratios and high bending stiffness. These honeycomb structures are close-celled with limited access into the core regions. The cores may be attached to the facesheets or plates by conventional joining methods, such as adhesive bonding, brazing, diffusion bonding and welding. Recently, lattice truss structures have been explored as an alternate cellular core topology. Pyramidal lattice truss structures are usually fabricated from high ductility alloys by folding a perforated metal sheet along the perforations, creating accordion-like structures. Conventional joining methods such as brazing or laser welding are then used to bond the core to solid facesheets, forming sandwich structures. The lattice topology, core relative density, and parent alloy mechanical properties, along with the bond strengths, determine the mode of truss deformation and, therefore, the out-of-plane and in-plane mechanical properties of these structures.
The design of the core-facesheet node interface is of the utmost importance. Ultimately, this dictates the maximum load that can be transferred from the facesheets to the core. Node bond failure has been identified as a failure mode for sandwich structures, especially metallic honeycombs. However, analogous node failure modes have been observed in sandwich panels utilizing tetragonal and pyramidal lattice truss cores during shear loading. Assuming sufficient core-faceplate bond (facesheet-bond) strength and ductility, when sandwich panels are subjected to intense shear or bending loads, the nodes transfer forces from the facesheets to the core members and the topology for a given core relative density dictates the load carrying capacity. When the node-facesheet interfacial strength is compromised by poor joint design or inadequate bonding methods, node bond failure occurs resulting in premature failure of the sandwich panel. Numerous factors determine the robustness of nodes, including joint composition, microstructure, degree of porosity, geometric effects (which control stress concentrations) and the nodes' contact area.
Micromechanical models for the stiffness and strength of pyramidal lattice truss cores, comprising elastic-plastic struts with perfect nodes have been recently developed. These models assumed that the trusses are connected to rigid face sheets and are of sufficiently low aspect ratio that bending effects make a negligible contribution to the stiffness and strength. These micromechanical models also assume the node strength is the same as the parent metal alloy. However, the measured elastic moduli rarely reach the predicted values because of variations in the length of the trusses and small initial departures from straightness introduced by manufacturing processes.
The design of the core-to-facesheet interface in honeycomb sandwich panels is of utmost importance. Ultimately, this dictates the amount of load that can be transferred from the face sheets to the core. This is even more critical for lattice-based cores since they can have a smaller node area than honeycombs of the same core density. Node bond failure has been identified as a key catastrophic failure mode for metallic honeycomb sandwich structures (See Bitzer, 1997). Similar node robustness problems have been observed in lattice-based sandwich structures. When sandwich panels are subjected to shear or bending loads, the nodes transfer forces from the facesheets to the core, assuming adequate node bond strength exists, and the topology for a given core relative density dictates the load carrying capacity. When the core-facesheet interface strength is compromised by poor joint design or weak bonding methods, node failure occurs and catastrophic failure of the sandwich panel results. Although numerous factors (including joint composition, microstructure, degree of porosity, and geometric constraints) determine the robustness of nodes, the node contact area serves as a critical limiting factor in determining the maximum force that can be transmitted across the core-facesheet interface.
Initial efforts to fabricate millimeter scale structures employed investment casting of high fluidity casting alloys such as copper/beryllium (See Wang et al., 2003), aluminum/silicon (See Deshpande et al., 2001, Deshpande and Fleck, 2001, Wallach and Gibson, 2001, Zhou et al., 2004), and silicon brass (See Deshpande and Fleck, 2001). Investment casting begins with the creation of a wax or polymer pattern of the lattice truss sandwich structure. The sandwich structure is attached to a system of liquid metal gates, runners, and risers that are made from a casting wax. The whole assembly is coated with ceramic casting slurry. The pattern is then removed and the empty (negative) pattern filled with liquid metal. After solidification, the ceramic, gates, and runners are removed, leaving behind a lattice based sandwich structure of homogeneous metal. However, the tortuosity of the lattices made it difficult to fabricate high-quality investment-cast structures at the low relative density (2-10%) needed to optimize sandwich panel constructions (See Chiras et al., 2002). In addition, the inherent low quality of as-cast metals resulted in sandwich structures that lacked the robustness required for the most demanding structural applications (See Sugimura, 2004).
The toughness of many wrought engineering alloys is evidenced by development of alternative fabrication approaches based upon perforated metal sheet folding (See Sypeck and Wadley, 2002). These folded truss structures could be bonded to each other or to facesheets by either transient liquid phase (TLP) bonding or micro welding techniques to form lattice-truss sandwich panels. Panels fabricated with tetrahedral (See Sypeck and Wadley, 2002, Rathbun et al., 2004, Lim and Kang, 2006) and pyramidal lattice-truss (See Zok et al., 2004, Queheillalt and Wadley, 2005, McShane et al., 2006, Radford, et al. 2006) topologies have been made by the folding and brazing/TLP bonding method. However, the node bond strength and the topology for a given core relative density may dictate the load-carrying capacity. While these structures are much more robust than their investment cast counterparts, their robustness may be dictated by the quality of the bond between the core and facesheets.
A detailed description of the fabrication approach for making 6061 aluminum alloy lattice truss structures can be found in Multifunctional Periodic Cellular Solids and the Method of Making the Same (PCT/US02/17942, filed Jun. 6, 2002), Method for Manufacture of Periodic Cellular Structure and Resulting Periodic Cellular Structure (PCT/US03/16844, filed May 29, 2003), and Methods for Manufacture of Multilayered Multifunctional Truss Structures and Related Structures there from (PCT/US2004/004608, filed Feb. 17, 2004), of which all of the PCT Applications are hereby incorporated by reference herein in their entirety. Briefly, these patents describe a folding process used to bend perforated sheets to create a single or multiple-layered lattice truss structures. The folding is accomplished using a paired punch and die tool or a finger break to fold node rows into the desired truss structure. The lattice truss core is then joined to facesheets via one of the previously mentioned methods to form the lattice truss sandwich structure (i.e. adhesives, welding, brazing, soldering, transient liquid phase sintering, etc.).